Were the byzantines still imperialistic in the middleages?

 I have never heard that byzantines had knights?!

To answer the first question, yes the Byzantine Empire was imperialistic (I think any nation which is an empire is, by definition, imperialistic). But they basically were a wealthy and well satisfied lot. They had a proud heritage and living standards, at least in Constantinople, typically vastly better than that of other peoples throughout most of their history. As a result they were more interested in maintaining a defensive structure to keep what they had. Situated between vast hordes of waiting opponents in the Balkans and the East, they knew that a defensive infrastucture was much safer and more viable than the granduer laden campaigns of such men like Justinian. But while they did engage in holding pretensions to certain regions such as Italy, their aggression was fairly piecemeal.

They did maintain the laws and government of the Roman Imperium, though with some modifications. The legal code became increasingly Christianized in its emphasis, drawing more and more on biblical teachings later in the Empire's history. Another notable work of legal reform was the Basilica, finally completed under scholarly Emperor Constantine VII. I would also argue the law of Byzantium became increasingly bureaucratic in response to the need to optimize use of resources in the face of growing threats to Imperial security. The farmer's law of Justinian II is a good example of this, as are plenty of other legal works which have survived.

As for the army, well in more than 11 centuries of existence it did change pretty radically. In its infancy growing up as de facto of the Western Empire it relied on mercenaries. In the late 5th century they made the first steps toward recruitment of native soldiers (Isaurians in this case). In the 7th century, with mercenary supplies drying up and being deprived of the income rich lands of Egypt and Syria, Byzantium radically transformed itself into a state which recruited its own citizens, had a very well organised militia force as well as a professional state army. Following Manzikert their army was an amalgam of mercenaries and locally recruited troops (typically of inferior quality to the ones recruited prior to Manzikert). Finally from the early 13th century onwards Byzantium fielded an army once again dominated by mercenaries, with locally recruited troops tending to fill in for emergency roles and being recognised as being of poor quality and low reliability.